The present invention relates generally to cutting inserts and, more particularly, to cutting inserts having radiused cutting edges.
The optimal sharpness of a cutting edge in a cutting tool often depends upon the nature of the material being cut. For softer material, a sharp edged tool may be preferable while, for harder materials, a sharp edge may be too easily broken. For harder materials, it is known to hone or round the cutting edge. However, too large of a hone can increase cutting forces and tool temperatures. Conventional wisdom is that the hone should be about one-third to one-half the size of the feed. See Kennedy, A Better Edge, Conicity Technologies (http://www/conicity.com/betteredge.htm).
Ordinarily, hones are applied uniformly along a tool's cutting edge. However, cutting conditions can vary greatly along a cutting edge. At the leading edge, the uncut chip thickness is heaviest and the edge requires maximum protection. At the tool's trailing edge, uncut chip thickness decreases to near zero while the hone remains the same size. Chip thickness at the trailing edge is smaller than the hone, so the cutting edge removes material inefficiently, which can increase friction, cutting forces, temperature, and wear rate. Conventional wisdom suggests that it is desirable that the edge radius should be smaller on the tool corner radius because the uncut chip thickness decreases along the corner radius. See Kennedy, A Better Edge, Conicity Technologies (http://www/conicity.com/betteredge.htm).
In those handful of tools that do provide cutting edges with edge radii that are larger near the corner, such as SU1060321, on which the preamble of claim 1 is based, US2001/0135406, or EP0654317, the insert geometries tend to be rather simplistic, with the radiused edges transitioning directly into flat top surfaces. Such inserts are not designed to facilitate chip removal, and can risk damage to more vulnerable portions of the cutting edge.
During metal cutting operations, such as those using cutting tools with replaceable cutting inserts, so-called “notch wear” can occur which will adversely affect the surface texture of the workpiece and eventually weaken the cutting edge. As explained in Modern Metal Cutting, A Practical Handbook, pp IV-15, IV-25 (1994 Sandvik Coromant), notch wear on the trailing edge is a typical adhesion wear but can to some extent also result from oxidation wear. The notch will be formed where the cutting edge and the material part. The wear is thus very localized at the end of the cut where air can get to the cutting zone. Notch wear on the leading edge is mechanical, often with harder materials. In addition to oxidation, the causes of notch wear can include excessive cutting speed and insufficient wear resistance, and remedies can include reducing cutting speed, use of more wear resistant grade cutting inserts, and special coatings. It is desirable to provide alternative techniques for controlling notch wear.
In accordance with an aspect of the present invention, a cutting insert comprises a cutting edge, the cutting edge being symmetrical about a centerline, the cutting edge having a first edge radius at a first point at the centerline of the cutting edge and a different, second edge radius at second points on opposite sides of and remote from the centerline of the cutting edge.